We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or "wavy" configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.

Properties that can now be achieved with advanced, blue indium gallium nitride light emitting diodes (LEDs) lead to their potential as replacements for existing infrastructure in general illumination, with important implications for efficient use of energy. Further advances in this technology will benefit from reexamination of the modes for incorporating this materials technology into lighting modules that manage light conversion, extraction, and distribution, in ways that minimize adverse thermal effects associated with operation, with packages that exploit the unique aspects of these light sources. We present here ideas in anisotropic etching, microscale device assembly/integration, and module configuration that address these challenges in unconventional ways. Various device demonstrations provide examples of the capabilities, including thin, flexible lighting "tapes" based on patterned phosphors and large collections of small light emitters on plastic substrates. Quantitative modeling and experimental evaluation of heat flow in such structures illustrates one particular, important aspect of their operation: small, distributed LEDs can be passively cooled simply by direct thermal transport through thin-film metallization used for electrical interconnect, providing an enhanced and scalable means to integrate these devices in modules for white light generation.gallium nitride | solid-state lighting | transfer printing I ndium gallium nitride-based (InGaN) blue light emitting diodes (LEDs) hold a dominant position in the rapidly growing solid-state lighting industry (1, 2). The materials and designs for the active components of these devices are increasingly well developed due to widespread research focus on these aspects over the last one and a half decades. Internal and external quantum efficiencies of greater than 70% (3) and 60% (3), respectively, with luminous efficacies larger than 200 lm∕W (4) and lifetimes of >50;000 h (5) are now possible. The efficacies (i.e., 249 lm∕W), exceed those of triphosphor fluorescent lamps (i.e., 90lm∕W), thereby making this technology an appealing choice for energy-efficient lighting systems (4). In particular, electricity consumption for lighting potentially could be cut in half using solid-state lighting (2). Although there remain opportunities for further improvements in these parameters, the emergence of LEDs into a ubiquitous technology for general illumination will rely critically on cost effective techniques for integrating the active materials into device packages, interconnecting them into modules, managing the accumulation of heat during their operation, and spatially homogenizing their light output at desired levels of chromaticity. Existing commercial methods use sophisticated, high-speed tools, but which are based on conceptually old procedures that exploit robotic systems to assemble material mechanically diced from a source wafer, with collections of bulk wires, lenses, and heat sinks in millimeter-scale packages, on a device-by-device basis, followed by separa...

No abstract

We show the polarization characteristics of gain-guided vertical-cavity surface emitting lasers are related to the temperature-dependent cavity optical resonance and laser gain spectral alignment. Simultaneous nearly degenerate orthogonal eigen polarization states are observed at and above lasing threshold. The partitioning of power between the linear polarization states is shown to depend on the relative spectral overlap of the cavity resonance of each state with the gain. Near the condition of cavity resonance/gain alignment, an abrupt switch in the dominant eigen polarization with a region of polarized output fluctuations is evident. Rotation of the eigen polarization directions relative to the crystal axes is also observed at temperatures where the gain is blue shifted from the cavity resonances.

No abstract